Dc-to-dc power converters are widely used to supply power to electronic devices, such as computers, printers, etc. Such dc-to-dc converters come in a variety of configurations for producing a desired output voltage from a source voltage. For example, a buck or step down power converter produces an output voltage that is less than the source voltage. A typical step down converter has high conversion efficiency, is relatively simple, and requires no output transformer. In addition, a small output filter may be used and the output voltage has relatively low ripple. A typical step down converter is relatively small in size and may therefore be used in embedded systems, such as personal computers.
A typical dc-to-dc step down converter in integrated circuit form is made by Harris Corporation, the assignee of the present invention, under the designation 45V/14A BUCK Power IC. The 45 V/14A BUCK Power IC produces a voltage of 5.1 to 12 volts.
The 45 V/14A BUCK Power IC includes a plurality of double-diffused vertical metal oxide semiconductor (DMOS) field effect transistors (FETs) connected in parallel. The DMOS transistors are operated via a gate driver, which, in turn, is connected to a pulse width modulation (PWM) circuit. The gate driver of the 45 V/14A BUCK Power IC includes a pair of bipolar transistors connected in a conventional totem pole configuration. One bipolar transistor pulls up the gate voltage of the DMOS transistors to thereby turn on the DMOS transistors. Similarly, the other bipolar transistor pulls down the gate voltage to thereby turn off the DMOS transistors to thereby regulate the output voltage.
As is typical, the 45 V/14A BUCK Power IC also includes circuitry for implementing soft-start features, as well as protection and status features. The soft-start works in conjunction with an external capacitor and the reference voltage/error amplifier to allow the output voltage to slowly ramp to its desired value during start-up. The protection circuitry includes over-voltage, under-voltage, over-current, and over-temperature protection.
The pulse width modulation (PWM) circuit of a typical dc-to-dc converter, such as the 45V/14A BUCK Power IC, accepts a clock signal as an input along with a signal from an error amplifier. The error amplifier, in turn, is connected to the output voltage and an internal precision voltage reference having a predetermined output voltage less than the output voltage. For example, for the 45 V/14A BUCK Power IC, the precision reference voltage is 5.1 volts. Accordingly, the output voltage of the power IC cannot be less than 5.1 volts.
There are many commercial applications where a relatively low dc voltage in the range of 2 to 5 volts is desired, such as for powering a microprocessor. Unfortunately, a step down power converter including DMOS transistors may not be suitable for such a low voltage application. For example, the DMOS transistors of the 45 V/14A BUCK Power IC have a turn on resistance which is relatively high (about 75 m.OMEGA.) and which may adversely effect the available output current and efficiency for such a low voltage application. In addition, since many thousands of DMOS transistors are provided on the integrated circuit die, reducing individual transistor cell size is also important.
A new generation of microprocessors, for example, may be screened device-by-device to obtain optimum performance and speed based on operating voltage. Unfortunately, most common power supplies are not precisely and readily reconfigurable to select a desired output voltage.